The experiments conducted in the Institute of Physical Chemistry of the
Polish Academy of Sciences on super-thin liquid-crystal films created on water
surface allowed the surprisingly slow and continuous rotational motion of
molecules, rotating "in unison", to be observed nearly with the naked
eye.

Scientists
from the Institute of Physical Chemistry of the Polish Academy of Sciencies
(IPC PAS) established that in the liquid-crystal layers that are several
nanometers thick and created on water surface, molecules may rotate with
extremely low speed, just one revolution per several minutes. Such slow
rotational motion is a real surprise since it was expected that rotation would
be quickly destroyed by thermal fluctuations. "The slowdown in rotation of
molecules is nothing unexpected in liquid crystals usually formed of thousands
of layers. However, in our experiments we have monolayers and we can observe
the effects of very slow rotational motion of chemical molecules nearly with
the naked eye,” emphasises Prof. Robert Hołyst from the IPC PAS.

The
experiment conducted in the IPC PAS is distantly related to famous experiments
carried out by Benjamin Franklin and connected with quieting agitated water by
spilling oil on it. During one of the attempts Franklin noticed that oil
spilled on the surface of a pond became so thin at a certain point that it
stopped spreading. "We do something similar but on a smaller scale: we
spill microlitres of liquid crystal on water surface. Its molecules form a
monolayer, that is a layer which is one-molecule thick,” explains PhD Andrzej
Żywociński from the IPC PAS.

Molecules of
the liquid crystals that are examined have amphiphilic character – the
hydrophilic group of a chain attaches to water surface, over which the
hydrophobic tail protrudes making dissolution impossible – and they freely move
across the water surface, which means they behave like gas in two-dimensional
space. However, researchers were interested in the behaviour of liquid crystals
in a liquid phase. A gas may be transformed into a liquid or solid as a result
of changes in temperature or pressure. If we use the latter, solidification is
achieved at high pressures of at least several dozens of atmospheres.
Fortunately, in the case of monolayers a suitably high pressure can be easily
obtained with a device called the Langmuir balance. It is a shallow tank filled
with water, with two hydrophilic barriers between which there is a film of the
liquid crystal several nanometers thick."It is sufficient to decrease the distance between the barriers in
order to achieve an increase in surface pressure that will cause the liquid crystal
to become liquid or even solid,” says Patrycja Nitoń, a PhD student from the
IPC PAS.

The surface
of a liquid-crystal film in a liquid phase was observed with the Brewster angle
microscope at relatively small enlargement. The Brewster angle is an angle at
which light falling on the surface of a dielectric reflects off fully linearly
polarized, which means that the component of the electric field oscillates in
one plane. If a polarizer is placed in the way of such polarized light so that
the light goes through it, it will stop the entire reflected light and under
the Brewster angle microscope clean water will look black. However, if on the
water surface there is something which twists the plane of polarization, bright
reflections will appear.

In the
experiments conducted in the Institute of Physical Chemistry of the PAS it was
analysed how a monolayer of a ferroelectric SmC* liquid crystal behaved on
water surface. It is typical for SmC* phase that molecules spontaneously
arrange themselves into layers and each subsequent layer is slightly twisted in
relation to the others. "We have only one layer which we can imagine to be
a forest of molecules inclined in the same direction at certain angle,"
explains PhD Żywociński. When water molecules evaporate, they hit into groups
of atoms of various sizes which are connected with an asymmetric (chiral)
carbon atom in each molecule of the liquid crystal. Due to the asymmetry, the
fragments of molecules of liquid crystals protruding over the water surface act
as the sails of a windmill and they start to rotate collectively (this effect
was for the first time observed by Prof. Hiroshi Yokoyama from Japan). A
molecule must be constructed in an appropriate way to be able to rotate. The
polar group that keeps it at the water surface should not be too big as it
would be immersed too deeply and would hinder the rotation caused by the
asymmetric chiral group which is hit by the molecules of evaporating water. The
chiral group, on the other hand, must remain distinctly above the surface.

Rotating
molecules change the polarization plane of the reflected light and in the field
of view of the Brewster angle microscope there are areas of periodically
changing brightness. The quickest rotation of molecules observed in this way lasted
five seconds and the slowest as long as eight minutes. It is probably possible
to achieve even slower rotational motion but not through the decrease of
temperature (as liquid crystals become solid then) but by saturating the air
with water vapour, which would decrease the pace of evaporation and thus the
frequency of collision of water molecules and "sails" of liquid
crystals.

Slowly rotating molecules of liquid crystals can
be used to construct nanodevices. "It is possible to construct a molecule
in which a group of atoms playing the role of a sail would be a kind of a
nanodrive. Then we would create a real molecular nanoengine driven by a water
vapour stream," says PhD Żywociński, and he adds that scientists are now
working on the possibility to transfer this collective rotation of single
molecules to larger objects.

The Institute of Physical
Chemistry of the Polish Academy of Sciences (http://www.ichf.edu.pl/) was
established in 1955 as one of the first chemical institutes of the PAS. The
Institute's scientific profile is strongly related to the newest global trends
in the development of physical chemistry and chemical physics. Scientific
research is conducted in nine scientific departments. CHEMIPAN R&D
Laboratories operating as part of the Institute implement, produce and
commercialise specialist chemical compounds to be used, in particular, in
agriculture and pharmacy. The Institute publishes approximately 300 original
research papers annually.

The effects of rotation of liquid-crystal
molecules in a monolayer. The marked area changes periodicallyfrom dark into bright, depending on how the
rotating molecules twist the polarization plane of the reflected light. False
colours. (Source: IPC PAS)

The film made at the Institute of Physical
Chemistry of the PAS shows water surface covered with a liquid crystal
monolayer. Rotating molecules change the polarization plane of the reflected
light and cause periodic changes in brightness, which are particularly well
noticeable in the spiral at the bottom of the monitor. The field of view is 4.8
x 6.4 mm. (Source: IPC PAS)